For the treatment of exhaust gases of combustion engines substrates having open or closed channels or combinations of those is used. In open channels the exhaust gas is directly flowing through the substrate. In substrate having closed channels exhaust gas is forced to flow through walls, e.g. through ceramic or metallic porous walls. In open channels the reduction of gaseous impurities is often high but the reduction of impurity particles is low, e.g. from 10 to 15%. In closed channels/wall flow filters the reduction of gaseous impurities is high and also the reduction of impurity particles is high, e.g. from 70 to 99%. The problem in closed channels is the clogging of walls. Closed channels/wall flow filters will gradually wholly clog if they are not cleaned. Pressure loss will also increase. One way to keep the channels open is to clean them continuously or periodically.